What is that cylinder on cables?
What's this part?
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It is a Ferrite Bead.
They're used to reduce and dissipate high frequency noise.
To prevent it from moving on the cable, it is overmoulded with plastic.
Functionally, it is a dissipative low-pass filter. It's basically an inductor with very high reluctance, so the magnetic field which it forms get dissipated as heat, rather than reflected back on the cable.
The device in your picture is most likely a ferrite ring and is used to reduce RF interference. You'll also see them on a lot of video cables.
You can also buy them as a separate item
Have a look here